src/Pure/pure_thy.ML

 fun thms_containing theory spec =
   (theory :: Theory.ancestors_of theory)
   |> map (fn thy =>
       FactIndex.find (fact_index_of thy) spec
       |> List.filter (fn (name, ths) => valid_thms theory (Name name, ths))
-      |> gen_distinct (eq_fst (op =)))
+      |> distinct (eq_fst (op =)))
   |> List.concat;
 
 fun thms_containing_consts thy consts =
   thms_containing thy (consts, []) |> map #2 |> List.concat
   |> map (fn th => (Thm.name_of_thm th, th));